With the ever-increasing amounts of information generated by computers and by other means, it becomes more of a problem to reduce that information to permanent visible form as printing. Also, to enable the reader to comprehend the printed data more readily, it has become desirable to print parts of the data in color as graphs and different color blocks of text. One preferred type of printing is ink jet printing.
Ink jet printing systems can be divided into continuous jet-type and drop-on-demand-type systems. In the former, a succession of ink drops is ejected from a small nozzle toward a recording medium such as a paper sheet. Selected ones of these drops are deflected electrostatically into a gutter; the remaining undeflected drops reach the paper and form the printed lines and characters thereon according to a standard dot matrix. In the drop-on-demand or impulse jet-type printer, the volume of a pressure chamber filled with ink is suddenly decreased by the impression of an electrical driving pulse whereby an ink droplet is jetted from a nozzle communicating with that chamber. Thus, a single drop of ink is transferred to the paper by a single driving pulse following which the system returns to its original state. During printing, a succession of such droplets is ejected in response to a succession of drive pulses to form a character or figure on the paper according to a predetermined dot matrix. While some aspects of the present invention have applicability to both types of jet printing, we will describe the present invention as it is applied to a drop-on-demand-type printer.
Also, in ink jet printing, three different types of inks are normally used, namely water-base inks, oil-base inks and hot-melt or wax-base inks. However, there are certain disadvantages to using the first two types of ink in ink jet printers. The main disadvantage is that water- and oil-base inks interact with the fibers of ordinary paper so that the quality and resolution of the printing are not as high as might be desired. For the same reason, the ink colors are muted so that the printing is not be as bright and vivid as desired. Some of these problems can be overcome by using specially treated or coated paper as the recording medium. However, that tends to be a rather expensive expedient and does not entirely solve these problems in any event.
It has been proposed to improve the quality of ink jet printing, including color printing, through the use of hot-melt or wax-base inks which do not interact with ordinary paper. Indeed, special hot-melt subtractive-color inks have actually been developed and tested in non-rotary ink jet printers, resulting in the production of high quality subtractive-color printing, examples of such inks being disclosed in application Serial No. 688,000, filed Dec. 31, 1984, owned by the assignee of the present application. However, these non-rotary printers employ printing heads which have to be translated back and forth across the recording medium in order to print the lines of text thereon so that considerable time and energy is wasted accelerating and decelerating the head.
In order to increase printing speed, it is also known to mount a multiplicity of ink jet printing nozzles around the periphery of a rotating print head or wheel and to wrap the paper sheet or other recording medium about the wheel opposite the nozzles. In this arrangement, different nozzles on the wheel are aimed at slightly different locations on the paper so that, when they are pulsed, in timed relation to the rotation of the wheel, a line or lines of character-forming dots can be printed on the paper during each revolution of the wheel. The wheel is rotated continuously and the paper is advanced continuously in a direction parallel to the head axis in timed relation to the rotation of the wheel so that the wheel prints successive lines of dot matrix characters on the paper. However, the prior rotary ink jet print heads of which we are aware can only print in one color and only using water- or oil-base inks; they are not suitable for printing in black and white as well as in color, much less using wax-base inks of the type disclosed in the aforementioned application.
It would be desirable therefore if there existed an ink jet print head of the rotary variety which could print on ordinary paper using hot-melt or wax-base printing inks. There is an additional need for a rotary ink jet print head which can print in black and white and in color, especially with such wax-base inks, in order to produce high quality color copy at the speed and efficiency demanded today by the printing and graphics industries.